1. Field of the Invention
The present invention relates to an electrophotographic photoconductor, more particularly to an electrophotographic photoconductor comprising a photoconductive layer and a protective layer formed thereon for protecting the photoconductive layer, which surface protective layer has excellent anti-peeling performance and is capable of maintaining electrophotographic characteristics of the photoconductor even when used repeatedly for an extended period of time.
2. Discussion of Background
Conventionally, as photoconductors for use in electrophotography, there are generally known a photoconductor comprising an electroconductive support and a photoconductive layer formed thereon, which comprises selenium or a selenium alloy as a main component; a photoconductor comprising a photoconductive layer, which comprises an inorganic photoconductive material such as zinc oxide or cadmium sulfide and a binder agent in which such an inorganic photoconductive material is dispersed; a photoconductor comprising a photoconductive layer, which comprises organic materials such as poly-N-vinylcarbazole and trinitrofluorenone or an azo pigment in combination; and a photoconductor comprising a photoconductive layer, which comprises an amorphous silicon-based material.
Generally, "electrophotography" is an image formation process. In electrophotography, the surface of a photoconductor is uniformly charged in the dark to a predetermined polarity, for instance, by corona charging. The uniformly charged surface of the photoconductor is then exposed to light images to selectively dissipate electric charges from the areas exposed to the light images, so that latent electrostatic images are formed on the surface of the photoconductor. The thus formed latent electrostatic images are developed into visible images by a developer comprising a coloring agent such as a dye or pigment, and a binder agent such as a polymeric material.
The photoconductor for use in such an electrophotographic process is required to have the following fundamental characteristics: (1) chargeability to an appropriate potential in the dark, (2) minimum dissipation of electrical charge in the dark, and (3) rapid dissipation of electrical charges from the areas exposed to light.
Recently, however, in accordance with the recent development of high speed and large size electrophotographic copying machines, in addition to the above-mentioned fundamental characteristics, high reliability is demanded for such a photoconductor as to the capability of forming images with high quality even if the photoconductor is repeatedly used for an extended period of time.
Causes for shortening the life of photoconductors for use in electrophotographic copying machines can be classified into the following two causes:
One cause is the photoconductor being frictioned, or scratches being formed on the surface of the photoconductor by mechanical stress being applied to the photoconductor while in use, in particular, in the course of a development process, a cleaning process or a copy paper transportation process.
The other cause is the photoconductor being chemically damaged, which is caused by corona charging in the course of a charging process, an image transfer process and a transfer sheet separation process.
As a technique of preventing the photoconductor from being frictioned, a method of providing a protective layer on the surface of the photoconductor is known. Specific examples of such a method include a method of providing an organic film on the surface of a photoconductor as disclosed in Japanese Patent Publication 38-15466; a method of coating the surface of a photoconductor with an inorganic oxide as disclosed in Japanese Patent Publication 43-14517; a method of providing an insulating layer on the surface of a photoconductor with an adhesive layer being interposed therebetween as disclosed in Japanese Patent Publication 43-27591; and methods of providing a-Si layer, a-S:N:H layer, a-Si:O:H layer or the like on the surface of a photoconductor by a plasma CVD method, a photo CVD method or the like as disclosed in Japanese Laid-Open Patent Applications 57-179859 and 59-58437.
Furthermore, recently films with high hardness consisting of carbon, or comprising carbon as a main component, which are collectively referred to as, for instance, a-C:H film, an amorphous carbon film or noncrystalline carbon film, or a diamond-like carbon film are produced by the plasma CVD method, the photo CVD method, a sputtering method, or the like, and the utilization of such films as a protective layer for a photoconductor has been actively proposed. For instance, Japanese Laid-Open Patent Application 60-249155 discloses the provision of a protective layer comprising amorphous carbon or carbon with high hardness on the surface of a photoconductive layer; Japanese Laid-Open Patent Application 61-255352 discloses the provision of a protective layer comprising a diamond-like carbon on the top surface of a photoconductive layer; Japanese Laid-Open Patent Application 61-264355 discloses the provision of an insulating layer with high hardness comprising carbon as a main component on a photo-conductive layer; and Japanese Laid-Open Patent Applications 63-220166, 63-220167, 63-220168 and 63-220169 disclose protective layers, each of which comprises a noncrystalline hydrocarbon film, which contains at least one element selected from the group consisting of nitrogen atom, hydrogen atom, a halogen atom, an alkali metal atom, and the like, and is formed by glow discharge.
These methods provide photoconductors with significantly improved surface hardness and excellent abrasion resistance. However, the thus obtained photoconductors do not have sufficient resistance against the peeling of the protective layers away from the surface of the photoconductors, which is caused by mechanical stress applied locally to the protective layers while in use for an extended period of time.
In order to improve the adhesion between a surface protective layer and a photoconductive layer, there has been proposed in Japanese Laid-Open Patent Application 1-22716 a photoconductor provided with first and second fluorine-containing, noncrystalline hydrocarbon films which are overlaid and serve as a surface protective layer, with the first film being in contact with the photoconductor and the second film being overlaid on the first film, and the concentration of the fluorine being higher in the first film than that in the second film. However, the peeling resistance of the protective layer of this photoconductor is still insufficient for use in practice.
Furthermore, when the above-mentioned photoconductors are repeatedly used in practice in the electrophotographic process, the surface potential of the areas on the surface of each of the photoconductors, which areas have been subjected to a first charging and then exposed to light images, tends to be increased either in the short-run or in the long-run. In other words, there is a tendency that the residual potential of such exposed areas in the surface of the photoconductor is built up, so that eventually, normal images cannot be obtained.
This indicates that the overall durabilities of the photoconductors are not improved by the provision of any of the above-mentioned protective layers.